1. Field of the Invention
The present invention relates to a surface acoustic wave device which is used as a resonator or a bandpass filter. Particularly, the present invention relates to a surface acoustic wave device including an IDT that is defined by a metal that is filled in grooves provided in a piezoelectric substrate.
2. Description of the Related Art
Surface acoustic wave devices have been widely used as a resonator and a bandpass filter. For example, WO2006/011417A1 discloses a surface acoustic wave device 1001 having a cross-sectional structure as schematically shown in FIG. 30. In the surface acoustic wave device 1001, a plurality of grooves 1002b is formed in an upper surface 1002a of a LiTaO3 substrate 1002. A metal is filled in the plurality of grooves 1002b, and an IDT 1003 having a plurality of electrode fingers is formed from the metal filled in the plurality of grooves 1002b. A SiO2 film 1004 is laminated so as to cover the upper surface 1002a of the LiTaO3 substrate 1002. Since the LiTaO3 substrate 1002 has a negative temperature coefficient of frequency TCF, the SiO2 film 1004 having a positive temperature coefficient of frequency TCF is laminated, and as a result, the absolute value of the temperature coefficient of frequency TCF of the surface acoustic wave device 1001 is decreased. In addition, since the IDT is formed using the metal filled in the plurality of grooves 1002b, a high reflection coefficient can be obtained in the IDT. In particular, when the wavelength of a surface acoustic wave is represented by λ, and when the thickness of Al filled in the grooves 1002b, that is, the thickness of the IDT made of Al, is set to 0.04λ, the reflection coefficient per electrode finger is 0.05, and a higher reflection coefficient can be obtained as the electrode thickness is increased.
Japanese Unexamined Patent Application Publication No. 2004-112748 discloses a surface acoustic wave device shown in FIG. 31. In a surface acoustic wave device 1101, an IDT 1103 is provided on a piezoelectric substrate 1102 made of LiTaO3 or LiNbO3. In addition, a protective film 1104 is arranged so as to cover the IDT 1103. Furthermore, in a remaining region other than a region in which the IDT 1103 and the protective film 1104 are provided, a first insulating layer 1105 made of SiO2 is formed to have a thickness equal to that of a laminated metal film in which the IDT 1103 and the protective film 1104 are laminated to each other. In addition, a second insulating layer 1106 made of SiO2 is laminated so as to cover the first insulating layer 1105. When a metal having a higher density than that of Al is used for the IDT 1103, the absolute value of the reflection coefficient can be increased, and undesirable ripples can be suppressed.
In the surface acoustic wave device 1001 described in WO2006/011417A1, as the thickness of the IDT made of Al is increased, the absolute value of the reflection coefficient can be increased. However, the inventors of the present invention discovered that superior resonance characteristics cannot be obtained by only increasing the absolute value of the reflection coefficient. That is, it was found that in the surface acoustic wave device described in WO2006/011417A1, although the absolute value of the reflection coefficient can be increased by increasing the thickness of the electrode made of Al, since the sign of the reflection coefficient is negative, many ripples are generated in a passband, and as a result, superior resonance characteristics cannot be obtained.
In WO2006/011417A1, the relationship between the thickness of the IDT and the reflection coefficient is described only with respect to the case in which the IDT made of Al is provided on the LiTaO3 substrate. In addition, paragraph [0129] of WO2006/011417A1 discloses that another metal, such as Au, may also be used to form the IDT. However, this description only discloses that when a LiNbO3 substrate is used, another metal, such as Au, may be used. That is, WO2006/011417A1 does not disclose that when a LiTaO3 substrate is used, the IDT may be formed using a metal other than Al.
On the other hand, Japanese Unexamined Patent Application Publication No. 2004-112748 discloses as described above that when the IDT made of a metal having a higher density than that of Al is used, the absolute value of the reflection coefficient can be increased. However, Japanese Unexamined Patent Application Publication No. 2004-112748 only discloses that ripples can be suppressed by increasing the absolute value of the reflection coefficient, and the sign of the reflection coefficient has not been considered. In addition, in the surface acoustic wave device described in Japanese Unexamined Patent Application Publication No. 2004-112748, there has been a problem of a low electromechanical coupling coefficient k2.